The f-number, also called the f-stop or the f-ratio, is an optical component’s focal length divided by its diameter:
where N is the f-number, ƒ is the focal length, and D is the diameter. Often, this is written as “f/N,” so for N=4, it is written “f/4.” This number represents the light collecting efficiency of an optical component. A smaller number means more light is collected, but it also increases the effects of aberrations. All of StellarNet’s gratings have an f-number of f/4, except for the BLACK-Comet, which has an f-number of f/2. The grating in the BLACK-Comet has been subsequently corrected for aberrations. F-number is related to the numerical aperture by the following equation:
where NA is the numerical aperture and it is assumed that the optic is in air (n=1). This approximation holds for most experimental conditions.
